1. Field of the Invention
The present invention relates to a zoom lens equipped with a focus adjusting mechanism and a zooming cam mechanism for varying a focal length of a zoom lens optical system.
2. Description of Related Art
Various types of zoom lenses are typically equipped with zooming cam mechanisms for moving a lens or lenses through a cam mechanism comprising circumferential cams and rectilinear guide slots. Such a zoom lens equipped with a zooming cam mechanism must have a multiple-barrel structure which comprises a cam barrel having zooming cam slots and a guide barrel having rectilinear guide slots inserted in each other so as to move linearly in an axial direction. A lens element or lens elements forming part of a zoom lens system are mounted in a lens holder barrel provided with cam followers. The lens holder barrel is inserted in the cam barrel or the guide barrel with the cam followers in slide engagement with the zooming cam slots and the rectilinear guide slots so as to move in an axial direction during relative rotation between the lens holder barrel and the cam barrel.
Some types of zoom lenses are equipped with focus adjusting mechanisms for adjusting focus of the zoom lens. Such a focus adjustment mechanism is used to make adjustment of an axial position or a movable range of a focusing lens element in which the zoom lens system is best focused based on resolusion of images at various focal lengths. A focus adjusting mechanism for performing the axial position adjustment of the focusing lens for best focus comprises a focus adjusting ring member having a cam face and a biasing spring for urging the cam face against an end of a lens holder barrel. Resolusion of an image formed by the zoom lens system is examined while rotating the focus adjusting ring member about an axis of the zoom lens system to move the lens holder barrel in an axial direction. The lens holder barrel is fixed in a position in which the zoom lens system forms a best focused image. Alternatively, the adjustment of a movable range of the focusing lens for best focus is performed by selectively bringing a screw pin into engagement with recesses formed in the lens holder barrel for different movable ranges. Another type of focus adjusting mechanism known from, for example, Japanese Patent No. 3498530 includes a spring installed in and biasing the lens holder barrel that is changeable in biasing force so as to adjust a lens position for best focus.
In the prior art zoom lenses in which a focus adjusting mechanisms is installed in a lens holder barrel that is disposed inside the cam barrel, the screw pin for focus adjustment is hidden by the cam barrel when the zoom lens assembled, so that it is impossible to access the screw pin for selecting axial positions or movable ranges of the focusing lens element for best focus from the outside of the zoom lens. Therefore, in order to perform focus adjustment of a zoom lens, it is essential to disassemble the zoom lens.